On Feb 14, 2013, at 3:29 PM, Christopher Webber <cweb...@dustycloud.org> wrote:
> Hello,
>
> I'm with the GNU MediaGoblin project. We're happily using SQLAlchemy,
> but we're running into a problem that's making it impossible for us to
> presently write unit tests for our plugins.
>
> The problem is that we'd like to use SQLAlchemy's built-in cascading
> deletion. However, we have plugins. Consider the following plugin,
> which adds this table:
>
>
> class VideoData(Base):
> __tablename__ = "video__mediadata"
>
> # The primary key *and* reference to the main media_entry
> media_entry = Column(Integer, ForeignKey('core__media_entries.id'),
> primary_key=True)
> get_media_entry = relationship("MediaEntry",
> backref=backref(BACKREF_NAME, uselist=False,
> cascade="all, delete-orphan"))
>
> width = Column(SmallInteger)
> height = Column(SmallInteger)
>
>
> now consider that we are running two sets of tests, one with the
> videodata plugin turned on, one without. With the foreign key setup
> above, if we do a MediaEntry().delete() and the plugin is *not* on,
> thus the table *doesn't* exist, it tries passing SQL to delete data
> from a table that doesn't exist at all. As you can imagine, this
> makes SQLAlchemy barf.
>
> So, between our unit tests we need to register and unregister
> relations between the tables depending on which plugins are loaded.
> However, it looks like there is no builtin way to do this. We tried
> doing metadata.remove(VideoData.__table__), but that did not fix it.
> Furthermore, we have no idea how one would add back a relation,
> either!
unfortunately, surgically removing elements of a mapping is not supported, the
only option is to remove mappings entirely.
With declarative, removal of mappings is one area that becomes more
inconvenient as you can't so easily re-map by just saying "mapper()" again.
My advise would be to unit test the VideoData plugin entirely separately from
the main model, and apply it only to an ad-hoc series of mappings that ensures
it's behavior is as expected (this can be done using an alternate Base, for
example) - basically more of a true unit test rather than an integration test.
Other than that you'd need to find a way to entirely reload your declarative
model in order to produce the class model and mappings again.
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